Packet data latency is one of the performance metrics that vendors, operators and also end-users (via speed test applications) regularly measure. Latency measurements are done in all phases of a radio access network system lifetime, such as when verifying a new software release or system component, when deploying a system, and when the system is in commercial operation.
One performance metric that guided the design of Long Term Evolution, LTE, was better latency than previous generations of Radio Access Technologies, RATs, defined by the Third Generation Partnership Project, 3GPP. LTE is also now recognized by the end-users to be a system that provides faster access to the Internet and lower data latencies than previous generations of mobile radio technologies.
Packet data latency is important not only for the perceived responsiveness of the system; it is also a parameter that indirectly influences the throughput of the system. Hyper-Text Transport Protocol/Transport Control Protocol, HTTP/TCP, is the dominant application and transport layer protocol suite used on the Internet today. According to HTTP Archive, http://httparchive.org/trends.php, the typical size of HTTP-based transactions over the Internet range from a few 10 s of Kbytes up to 1 Mbyte. In this size range, the TCP slow start period is a significant part of the total transport period of the packet stream. During TCP slow start, the “congestion window” used by TCP for defining the amount of traffic that can be outstanding, i.e., transmitted but not acknowledged, and packet latency limits how quickly the congestion window can be optimized. Hence, improved latency improves the average throughput for these types of TCP-based data transactions.
Radio resource efficiency in general is positively impacted by latency reductions. Lower packet data latency could increase the number of transmissions possible within a certain delay bound; hence higher BLER targets could be used for the data transmissions freeing up radio resources potentially improving the capacity of the system. There are a number of current applications that will be positively impacted by reduced latency in terms of increased perceived quality of experience: examples are gaming, real-time applications like VoLTE/OTT VoIP and multi-party video conferencing. In the future, there will be a number of new applications that will be more delay critical. Examples may be remote control/driving of vehicles, augmented reality applications in e.g. smart glasses, or specific machine communications requiring low latency.
Finally, it is appreciated herein that reduced latency of data transport may also indirectly give faster radio control plane procedures like call set-up/bearer set-up, due to the faster transport of higher-layer control signaling. For example, the LTE air interface is based on radio access network control and scheduling. These facts impact the latency performance because a transmission of data depends on a round trip of lower-layer control signaling.
Consider FIG. 1, which illustrates example control signaling timing for scheduling requests. The data is created by higher layers at T0, then the “User Equipment” or UE modem needs to send a scheduling request (SR) to the base station (an “eNB” in LTE). The eNB processes the SR and responds with a grant so the data transfer can start at T6 in the figure.
Correspondingly, it is recognized herein that one area to address when it comes to packet latency reductions is the reduction of the transport time of data and control signaling, which, according to the teachings herein, can be accomplished by allowing for shorter scheduling intervals. Here, a “scheduling interval” is the smallest unit of time allocated when scheduling resources. In LTE, scheduling intervals are referred to as “Transmission Time Intervals” or TTIs. Each TTI spans one sub-frame, which in turn spans two slots, each slot comprising six or seven OFDM symbol times in dependence on whether normal or extended Cyclic Prefixes (CP) are in use. The reduction of the processing time of control signaling at the involved receiver, e.g., the processing time needed by a UE to process a scheduling grant, is also recognized herein as an important aspect of reducing latency.
Because the time needed for turbo decoding depends on the code block size, latency can be reduced by reducing the code block size. Hence, if the code block size (or equivalently the transport block size) is reduced, the decoding result will be available earlier (for a given decoding capability in terms of the number of parallel decoders). If instead of transmitting a single large code block of length 6000 bits once every 1 ms, one transmits two consecutive blocks of length 3000 bits every 0.5 ms, one may roughly to a first order halve the decoding latency for each block while still sustaining the bit rate. It should be noted that some performance degradations may attend this approach, as a consequence of using shorter block lengths, and there may thus be a tradeoff between receiver performance and latency reductions.
According to Release 8 of the LTE specifications, when a UE makes a scheduled uplink data transmission on the Physical Uplink Shared Channel or PUSCH, the transmission includes an uplink reference signal transmission in the middle of each slot spanned by the PUSCH subframe. One sees this arrangement in FIG. 2, which depicts the transmission of a DeModulation Reference Symbol or DMRS in the middle of each slot. In more detail, the vertical blocks in FIG. 2 correspond to individual symbol times—14 OFDM symbol times are shown, corresponding to two slots of 7 symbols per slot. In FIG. 2, each pair of slots constitutes a subframe and FIG. 3 depicts two such subframes.